1. Field of the Invention
This invention relates generally to data disk drive systems and more specifically to servo systems for disk drive systems which use flexible disks.
2. Description of the Prior Art
In electronic computer technology, it is common to store data in binary form on the face of a rotatable disk. The face of the disk is coated with a magnetizable substance such as iron oxide. The disks are operated by rotating them like phonograph records and the binary data is encoded upon, or retrieved from, the face of the disk by a movable magnetic transducer device called a read/write or transducer head. The binary information is encoded on the face of the disk in concentric rings, called tracks, and the read/write head can move radially along the disk face to select a particular track to record or retrieve information. The data disks can be rigid or flexible.
These rigid disks typically have data densities of about five hundred tracks per inch of radius of the disk. The flexible disks typically have densities of forty-eight or ninety-six tracks per inch. Because of the high density, precise positioning of the read/write head is necessary so that the head can accurately gain access to a particular desired track on the surface of the disk.
One method of obtaining precise positioning is described in the co-pending application for a "Servo Synchronization Method and Apparatus for a Magnetic Disk" by Hubert Song which is assigned to the same assignee. In this system, the disks have servo sectors which alternate with data sectors. The servo sector tracks contain positioning data to help the transducer head stay on the data track. The servo tracks are radially off-set from the data tracks such that a transducer head passes between two servo tracks when it is positioned along a data track.
The consecutive servo tracks alternate between having an "A" burst and a "B" burst. The transducer head reads the "A" and "B" bursts from the servo tracks on either side. The intensities of the "A" and "B" bursts are measured and the head is adjusted to keep the head midway between the servo tracks and directly on the data track.
One problem with this type of servo system is that there are a limited number of servo sectors per revolution of the disk. The servo system of the disk drive thus has only a limited number of position samples it can make per revolution. Few samples means that it is harder to keep the head on track, especially on high track density disks. One solution is to include more servo sectors on each disk and thus increase the amount of position sampling per revolution of the disk. However, an increase in the number of servo sectors leads to a corresponding decrease in the space available for storing data in the data sectors.
Another problem with servo systems of the prior art involves misalignment of the disks. The disks may not be perfectly centered about their spindle hole when they are clamped in the disk drive. If the disk is off center, the disk track will oscillate radially inward and outward once per revolution. For a disk rotating at six hundred rpms, this oscillation occurs ten times per second or ten hertz. The head is harder to keep on track when this occurs.
A similar problem is caused by disks which become noncircular in shape due to the temperature or humidity. These oval shaped disks will have tracks which oscillate radially inward and outward twice per revolution. For a disk rotating at six hundred rpms, this oscillation occurs twenty times per second or twenty hertz.
Servo systems with linear coil motors have problems when the disk drive is tilted at an angle to the horizontal. The servo systems of the prior art are not able to adjust to provide increased force necessary to keep the head on track when the disk drive is tilted. Prior art systems experience trouble keeping the head on track with tilts of as little as five degrees.